(i) Field of the Invention:
This invention relates to a hydraulic servo for a friction coupling device of automatic transmission.
(ii) Description of the Prior Art:
Heretofore, hydraulic servos for friction coupling devices or elements of automatic transmission have usually included an annular drum having an outer cyliner, an inner cylinder and a side wall connecting the two cylinders together, a piston fitted between the outer and inner cylinders of the drum, and return springs mounted on the outer side of the outer cylinder or on a wall portion of the piston, and adapted to engage a friction coupling device upon supplying a line pressure between the drum and piston and to release the friction coupling device upon draining the line pressure. In a case where return springs are located in positions outward of the outer cylinder, however, the radial dimensions of the hydraulic servo of the friction coupling device become unduly large. On the other hand, when the return springs are mounted on a wall portion of the piston, the hydraulic servo of the friction coupling device has undesirably large dimensions in the axial direction. Further, when upshifting a vehicular automatic transmission, for example, from first to second speed, a friction brake B2 which holds the outer race F2 of a one-way clutch stationary in the engaged state is fixed to the autmatic transmission case. However, a hydraulic servo B-2 which operates to engage and release the brake B2 usually consists of a hydraulic drum and a piston with return springs, and has a large piston mounting area due to the existence of a broad space between the outer and inner cylinders of the hydraulic drum. Therefore, there is no space for mounting the return springs unless the case is extended to provide extra space for mounting the return springs. In addition, owing to the large dimensions of the piston, shift shocks occur at the time of upshift from first to second speed.